GDScript: Ignore unused arguments/local vars that start with _

Makes it simple to ignore particular arguments without adding special
comments, especially in engine-defined functions.

modules/gdscript/gdscript_parser.cpp

@@ -7397,7 +7397,7 @@ void GDScriptParser::_check_function_types(FunctionNode *p_function) {
 			}
 		}
 #ifdef DEBUG_ENABLED
-		if (p_function->arguments_usage[i] == 0) {
+		if (p_function->arguments_usage[i] == 0 && !p_function->arguments[i].operator String().begins_with("_")) {
 			_add_warning(GDScriptWarning::UNUSED_ARGUMENT, p_function->line, p_function->name, p_function->arguments[i].operator String());
 		}
 #endif // DEBUG_ENABLED
@@ -7855,10 +7855,12 @@ void GDScriptParser::_check_block_types(BlockNode *p_block) {
 	// Warnings check
 	for (Map<StringName, LocalVarNode *>::Element *E = p_block->variables.front(); E; E = E->next()) {
 		LocalVarNode *lv = E->get();
-		if (lv->usages == 0) {
-			_add_warning(GDScriptWarning::UNUSED_VARIABLE, lv->line, lv->name);
-		} else if (lv->assignments == 0) {
-			_add_warning(GDScriptWarning::UNASSIGNED_VARIABLE, lv->line, lv->name);
+		if (!lv->name.operator String().begins_with("_")) {
+			if (lv->usages == 0) {
+				_add_warning(GDScriptWarning::UNUSED_VARIABLE, lv->line, lv->name);
+			} else if (lv->assignments == 0) {
+				_add_warning(GDScriptWarning::UNASSIGNED_VARIABLE, lv->line, lv->name);
+			}
 		}
 	}
 #endif // DEBUG_ENABLED